The Pursuit of the Pankera: A Parallel Novel About Parallel Universes. According to the publisher, it was written in 1977 but not published due to copyright issues. The publisher doesn't say what the copyright issues are, however. I thought that they might involve L. Frank Baum's estate, since Heinlein was a big fan of the Oz books. In scrolling down to the reviews written by people who've bought and read the book, it turns out that the issues would have been with the estates of Edgar Rice Burroughs and E.E. "Doc" Smith. The first chapter of this novel, it seems, is the first chapter of The Number of the Beast, and then Heinlein veers off in a different direction (though keeping many of the characters the same). This review, however, gives me pause at the thought of reading it
I didn't end up enjoying TNOTB as I recall. There was a lot to like about the premise in the beginning but it ended up being very ponderous. I'd assume this would be too. If they find a "lost" manuscript that interests me, it's gonna need to be from before 1970
